Here is another excerpt from an upcoming study on 1 Thessalonians. This is a rough draft.
1 Thessalonians 3:5
5 For this reason, when I was not able to bear it any longer, I sent in order to find out about your faith, lest in some way, the tempter had tempted you and our labor may have been in vain.
5 Paul begins the last sentence with διὰ τοῦτο (For this reason) which is referring to “afflictions” that the Thessalonians were experiencing and why Paul wanted to really send Timothy. Bruce, on the other hand, sees διὰ τοῦτο (For this reason) pointing forward as Paul wanted to know about the Thessalonians’ faith (Bruce, WBC, 63). Both, in a sense, are probably correct as one thing always leads to another. Paul then moves from the 3rd person to the 1st person by saying κἀγὼ μηκέτι στέγων (when I was not able to bear it any longer). The phrase is somewhat of a repeat from verse 1 except Paul is now stating that he himself was the one who couldn’t stand not knowing what was going on with the Thessalonians. Were they faithful? Were they enduring? Were they holding on to their faith while suffering afflictions from their own fellow-citizens? Paul wanted to know so he sent Timothy in to find out! Paul uses εἰς τὸ γνῶναι τὴν πίστιν ὑμῶν (in order to know your faith). The construction indicates that the action of sending Timothy back to Thessalonica would result in Paul understanding the Thessalonians’ faith. He wanted to find out if they were still holding strong.
Then Paul “lets the cat out of the bag” so to speak by saying μή πως ἐπείρασεν ὑμᾶς ὁ πειράζων (lest in some way, the tempter had tempted you). The “tempter” of course being Satan. Paul has already referred to Satan in 2:19, but here Paul was concerned that “the temper” may have been taking advantage of the situation where the Thessalonians, being persecuted, could have been vulnerable to go back to their idolatrous ways in order to escape the persecution. If that had been the case, then the Apostles’ work would have been in vain or to no effect. For if “the tempter” used the Thessalonians’ situation in order to turn them back to idolatry, then Paul and company truly had “labored in vain”. Paul’s usage of καὶ εἰς κενὸν γένηται ὁ κόπος ἡμῶν (and our labor may have been in vain) comes from Isaiah 65:23a LXX:
23a οἱ δὲ ἐκλεκτοί μου οὐ κοπιάσουσιν εἰς κενὸν...
23a and my chosen ones will not labor in vain...
Although Isaiah 65:23a is positive, Paul places it in a negative context here (Fee, NICNT, 120). Paul’s use of εἰς κενὸν (in vain) is found throughout his letters (1 Corinthians 15:58; Philippians 2:16; Galatians 2:2; 2 Corinthians 6:1).
Although there is debate over such notions as one losing one’s place in God and thus losing one’s salvation, from this context, Paul certainly thought that it was possible (Fee, NICNT, 120), but there is a big difference between a person sinning because of one’s fallen nature and completely turning away from one’s faith. The tempter’s (Satan’s) main concern for tempting is to not just get christians to commit some sins, but to get them to completely turn their back on their christian faith (Green, PNTC, 164-165).